Troubleshooting
If Wicd fails to connect after you install it, make sure that the only entry in your /etc/network/interfaces file is
auto lo
iface lo inet loopback
You can change the contents of this file by pressing alt + f2, then typing gksudo gedit /etc/network/interfaces if you are using Gnome, or replace gedit with kate if you are using KDE. This will allow you to view and edit the /etc/network/interfaces.You can ignore lines that start with #.
Installing Wicd in Ubuntu
Installing Wicd in Ubuntu is very simple. You just have to add the Wicd repository to the Ubuntu package manager. To open the package manager in Gnome, go to Administration > Synaptic Package Manager. When it appears, go to Settings > Repositories > Third Party Software > Add..., and enter the following line:
deb http://apt.wicd.net gutsy extras
where gutsy is your version of Ubuntu in lowercase (dapper, edgy, feisty, gutsy, hardy). Then, click Reload, and wait while the package lists are downloaded. Now, search for "Wicd", and right click on it. Select Install, then press Apply, and Wicd will automatically be downloaded and installed for you. This will also keep you automatically up to date with the latest and greatest version of Wicd. Please note that this will remove network-manager, which is the default GNOME network manager and may cause loss of network connection temporarily.
In GNOME, to get the tray icon to automatically appear at boot, go to System > Preferences > Sessions. In the "Startup Programs" tab, click the "New" button. Give it a name ("Wicd" works fine). For the command, enter "/opt/wicd/tray.py".
In KDE, you'll need to make a desktop file, so see this post for the directions on how to fix it up.
Installing Wicd in Debian
You can also use the apt repository. Just add the following line to your /etc/apt/sources.list
deb http://apt.wicd.net debian extras
Now you can apt-get update and apt-get install wicd to install Wicd.
Installing Wicd in Arch Linux
Wicd is in the extra and testing repositories, so all that needs to be done is:
pacman -Sy wicd
It is recommended to take a look at the Arch Wiki page on Wicd for other notes.
Installing Wicd in Fedora
Thanks to an anonymous person(s), the wiki has instructions on installing Wicd on Fedora 7.
Installing Wicd in Slackware
Thanks to the hard work of Robby Workman, Wicd packages for Slackware can be obtained from his website.
Other Distros
If your computer does not have any internet connection, or you wish to download Wicd's source, you may visit the Release page and download the .deb or the .tar.bz2 source.